#siltstone

The ''Two Dog Palette,'' Predynastic Period ca. 3300-3100 BC,

The both sides of the two dog palette are carved in low relief with scenes depicting the frenzy of an animal hunt.

The imagery includes real animals, such as the pair of Cape hunting dogs that frame the top (from which the palette takes its name) and a mixture of fabulous animals.

Made of siltstone, 42.5 x 22 cm.

From Hierakonpolis (Nekhen).

Egyptian Research Account excavations, 1898.

There's new(ish) video up on youtube giving a quick guide to sandstones. This is the same video I uploaded last year, but I've cleaned the audio up so that you can actually hear what I'm saying now!

You can check it out below

Every rock that crops out of the ground is a record of the past, and so, in one way or the other it tells something about the environment in which it was formed. For example, the dark grey rock with fine and smooth texture you can see on the photo, looks that way because it formed on deep water sea bottom, where waves did not disturb the sediment that make up this rock, nor coarser sandy material was brought into it. This was a clam environment which allowed the sea-bottom animals to feed on the organic matter within the sediment (hence the dark colour) and is the reason why there are so many fossils in it, which I showed in the ‘underwater lilies’ post.

So, it is all good, and all makes sense!

But, then there are those large pieces of light grey rock (not shown, but some bigger than sedan car) with rough texture and little holes. Like you can see, its pieces are larger than anything else in a vicinity and are scattered all throughout the dark grey rock. This light grey rock does not belong to the sedimentary environment it is in, it got to where it is by other means than the sediment that surrounds it. And so, the plot thickens…

The Nankoweap formation.

The Nankoweap formation is the next unit up in the Grand Canyon, found atop the last unit – the Cardenas basalt. The Cardenas can be spotted near the bottom in this wide-angle view of the Canyon. See the black unit that has a dip to it, beneath the flat lying and younger sediments? That’s the Cardenas basalt. The layers on top? That’s the Nankoweap formation.

The Nankoweap formation sits on top of the big thick Cardenas basalt. The boundary between the two units is an unconformity; there are places where the basalt was eroded before sediments started depositing on top. There are also faults which cut through portions of the Cardenas basalt but do not cut the Nankoweap formation, making this unit clearly younger.

The rocks of this formation are sandstones with hematite cement, giving them a reddish color. The lower part of the Nankoweap is fairly finely-bedded while the upper portion has coarser beds.

Paleomagneticists have worked on this unit and found that the lower and upper portions of the Nankoweap record strongly different magnetic directions. This change implies that there is a lot of time missing in the middle of the Nankoweap – a disconformity, an unconformity that sits somewhere but doesn’t have major changes in rock type. The Nankoweap is younger than the Cardenas; zircons present in this unit are as young as 780 million years old, meaning the Nankoweap formation must be younger than that age. Its hard to tell when Nankoweap deposition started but this unit could potentially represent over 100 million years of time, and isotope changes within the unit suggest parts of it were being deposited 750 million years ago.

Like the quartzite below, this rock is composed of sandy sediments derived from erosion of the surrounding areas. There are some pieces of the Cardenas basalt in the Nankoweap, mixed with quartz and feldspar grains. The Nankoweap formation contains a variety of sedimentary structures like mud cracks and ripple marks, indicating that it was deposited in a setting like an arid lake or arid shoreline where the water would appear and disappear as precipitation patterns changed.

-JBB

Image credits: https://upload.wikimedia.org/wikipedia/commons/e/ea/View_from_Lipan_Point.jpg http://en.wikipedia.org/wiki/Cardenas_Basalt#mediaviewer/File:Grand_Canyon_landscape.jpg

Sources: http://bulletin.geoscienceworld.org/content/117/11-12/1573.full https://www.researchgate.net/publication/313843847_Synthesis_of_the_780-740_Ma_Chuar_Uinta_Mountain_and_Pahrump_ChUMP_groups_western_USA_Implications_for_Laurentia-wide_cratonic_marine_basins